The present invention relates to a high electron mobility transistor (HEMT), more particularly to a GaN-based HEMT.
It is strongly expected that a nitrogen-compound field-effect transistor using GaN serves as a power element to be operated at a high power and at a high frequency. The nitrogen-compound field-effect transistors which have been proposed are a Schottky gate field-effect transistor, MESFET (metal semiconductor field-effect transistor), HEMT or MODFET (modulated doped field-effect transistor), and MISFET (metal insulator semiconductor field-effect transistor). Of them, a GaN-based HEMT employing AlxGa(1-X)N as an electron supply layer is considered as a promising high power element since an electron concentration can be rendered higher than that of the GaAs-based HEMT. However, a conventional GaN-based HEMT has a problem in that a kink phenomenon sometimes occurs in the drain-current/voltage characteristics. If the kink phenomenon occurs, a power-added efficiency decreases in a large signal operation performed at a high frequency. The power-added efficiency η is defined as η=(Pout−Pin)/VdId, wherein Pout is an output power, Pin is an input power, Vd is a supply voltage and Id is a drain current. In addition, the distortion increases and the linearity deteriorates.
Now, the reason why the kink phenomenon occurs in the GaN-based HEMT will be explained. FIG. 1 is a schematic cross-sectional view of the GaN-based HEMT according to a first conventional example. In FIG. 1, reference numerals 11, 12, 13, 14, and 15 denote a GaN electron accumulation layer, AlxGa(1-x)N spacer layer, n-type AlxGa(1-x)N electron supply layer, AlxGa(1-x)N cap layer, and a sapphire substrate, respectively. Furthermore, a gate electrode 16 is formed on the cap layer 14, while a source electrode 17 and a drain electrode 18 are formed on the electron supply layer 13.
In the GaN-based HEMT according to the first conventional example, when a drain voltage increases to raise the intensity of the electric field within the electron accumulation layer 11, a current of electrons flows through a strong electric field region between the source electrode 17 and the drain electrode 18. As a result, pairs 22 of electrons and holes are generated by impact ionization within the electron accumulation layer 11. The electrons thus generated flow into the drain electrode 18, increasing the drain current a little. However, the effect of the increased drain current is small. On the other hand, the generated holes 23 are accumulated in a lower portion of the electron accumulating layer 11 as shown in the figure, due to the absence of the electrode for absorbing the holes. The potential of the electron accumulation layer therefore decreases, with the result that the drain current substantially increases in a drain-current saturation region of a graph showing the drain current/voltage characteristics. The drain current significantly increased in this way causes the kink phenomenon shown in FIG. 2.
FIG. 3 is a schematic cross-sectional view of a GaAs-based HEMT according to a second conventional example.
Reference numerals 11′, 12′, 13′, 14′, and 15′ of FIG. 3 are a GaAs electron accumulation layer, AlxGa(1-x)As spacer layer, n-type AlxGa(1-x)As electron supply layer, AlxGa(1-x)As cap layer, and GaAs substrate, respectively. Furthermore, a gate electrode 16′ is formed on the cap layer 14′, while a source electrode 17′ and a drain electrode 18′ are formed on the electron supply layer 13′.
In the GaAs-based HEMT according to the second conventional example pairs 22 of electrons and holes are also generated in the electron accumulation layer 11′ by the impact ionization as described in the first conventional example. However, most of the holes are absorbed by the gate electrode as shown in FIG. 3. Therefore, the holes are not accumulated in the electron accumulation layer 11′. As a result, the kink phenomenon, a problem of the GaN-based HEMT of the first conventional example, does not occur in the GaAs-based HEMT in the second conventional example.
The big difference of the GaN-based HEMT of the first conventional example from the GaAs-based HEMT of the second conventional example resides in that a large amount of piezoelectric polarization charges 21 are generated in a hetero-junction interface in the former GaN-based HEMT. This is because the ratio between GaN and AlxGa(1-x)N in lattice constant is larger than that between GaAs and AlxGa(1-x)As by an order of magnitude.
When the hetero junction of the GaN layer and the AlGaN layer is formed, positive charges are accumulated in the AlGaN layer near the interface at a GaN-layer side, whereas negative charges are accumulated in the AlGaN layer near the interface at a gate-electrode side due to the piezoelectric polarization effect. As a result, most of the holes generated by the impact ionization are prevented from flowing into the gate electrode by the piezoelectric polarization charges (positive charges) accumulated in the AlGaN layer near the interface at the GaN layer side. The holes are therefore accumulated in the GaN electron accumulation layer, causing the kink phenomenon.